Stabilization of microwave oscillators



| E. NORTON 2,602,897

July 8, 1952 STABILIZATIQN oF MICROWAVE OSCILLATORS 2 SHEETS- SHEET 2Filed Sept. 18, 1948 "Af/,YER ruw/l5. MIXER HIP IIIIIF b F/Pfa/ammfleF4670@ of 2 INVENTOR Z0 wel] E /Volion BY @gm-.f

ATTORNEY Patented July 8, 1952 sTABILIzATIoN F MICROWAVE osCILLA'roRs.'Lowell E. Norton, Princeton, N. J., assignor .to Radio Corporation ofAmerica, a corporation of Delaware iippiitajtionsememher18,1948,semaine.49.934,.l f

i f 19 =Claims.

This invention relates to methods and systems for stabilizing thefrequency of microwave voscillators by utilization of sharp molecularresonance exhibited bycertaingases at microwave frequencies." Themicrowave absorption spectra of certain gases, including ammonia,carbonyl sulphide and methyl halides, comprise lines of distinctivefrequency distribution for thejdiflerent gases. At very low pressures,these lines, orjabsorption regions, may break' uplinto' a plurality ofsharply dened lines, each'corresponding with,V a sharp molecularresonancel characteristic unaffected by ambient temperature, pressureand other common variables. i

In accordance with-thev present invention, the carrier frequency ofK themicrowave oscillator is modulated to produce side-band frequencies, atleast two `of which'are on opposite` slopes of one molecular resonancecharacteristic' selected vto correspond with ,the ldesired operatingfrequency of the microwave oscillator. Inv theinpassage through the'ga's,A pair of side-bandirequencies is differentially varied inamplitude and phase,4 in accordance-with the sense and extent ofydeviation from the* desired operating frequency. The resultingdifference in amplitude or phaseof the side-bandfrequencies is utilizedto vary lthe frequency of the microwave oscillator in compensation forits'deviation.

More particularly, in some forms of the invention the output ofthemicrowave oscillator is impressed upon two channels, each including amixer upon which the outputs of the'microwave oscillator and alow-frequency oscillator are impressed; A resulting` side-band frequencyin each channel is'imp'ressed 1,1Don`a vgas cell, the gas cellsdifferentially varying the amplitude or phase of the selected side bandswhich are thereafter vdemodulated and the resultant outputs of thechannels utilized as anerror control voltage for the microwaveoscillator.

' In otherforms of the invention, there is ineluded in one of thechannels, beyond the gas cell, a mixer upon which are impressed thelower side-band frequency'anda beat-frequency equal to theV differencebetweenithe side-band frequencies so to providethat thelout'putspofl'the channels shalll include componentsv having` the samefrequency but with 'a 'phase-'difference varyingyinaccordance with thefrequency-devia..

tion ofthe `,microwave' oscillator; in a preferred fo'rm,rtheldifference-frequencyis 'produced by impressing the outputs,ofthefmodulating oscillators uponajmixer whosepoutput,v in turn, is

applied to* a mixer upon which is'v also impressed the lower side-bandfrequency.`

In still another modification of the invention. the output of the:microwave oscillator is applied to a balanced modulator in advance ofthe gas cell and upon which is also impressed the output of alow-frequency modulating oscillator to Vproduce side-bands onoppositeslopes of a molecular resonance characteristic of the gas. Theoutput of the gas cellV is heterodynedwith oscillations of frequencycorrespondingwith the difference between the side-band frequencies so toproduce two equal frequencies of phase-difference varying in dependenceupon deviations from the desiredl operating frequency of the microwaveoscillator.

The invention further resides in methods and systems having the featureshereinafter described and claimed. y Y

For a more detailed understandingv of the invention and for.illustration of V'systems utilizing it, reference is made to theaccompanying drawings,inwhich: "i

Figure 1v is'aj block diagram of ai stabilized oscillator system [usingtwo control channels whose outputs are impressed upon anamplitudecomparison network for control of the oscillator frequency;

Figure 2 is an explanatory figure referred to in discussion of theoperation of the system of Figure l;

Figure 3 is a block diagram of a modification of Figure 1 in which theoutputs of the two control channels are impressed'upon aphase-comparison network;

Figure 4 is a schematic diagram of part of the phase-comparison networkof Figurev 3;;

Figure 5 is a modification ofthe system' of Figure3; and

Figure 6 is a modification utilizing a balanced modulator. v As morefully discussed in my copending applications, Serial Numbers 8,246,"6,975,` and 35,185, now Patent No. 2,584,608, many gasesunder reducedpressure exhibit alnelineabsorption, effect at microwave frequencies;Yin the,y case of ammonia, for example, many sharp resonances occurwithin a comparatively narrow frequency range, in the neighborhood of'agfrequ'ency -corresponding with 1.25 centimeters wavelength.

In the system shown in Figure 1, the microwave oscillator IIJ is to bestabilized at a selected one of these molecularresonance frequencies'.

like type. The mixer IIA therefore generates side-band frequencies 2Tand .2

which are propagated in both directions from the mixer in channel A.However, an attenuator I4A connected between mixer IIA and theoscillator IU prevents interaction of the side-band propagation from1Vmixer IIA with side-band propagation from mixer IIB in channel B. Animpedance-matching transformer 13A, of any suitable type for microwavefrequencies, is preferably connected between the mixer IIA and theattenuator I4A. n n

The frequency of modulating oscillator I2A is so chosen that one oftheaforesaid side-band frequencies fallson one slope of the chosen gasline exemplified, on. expanded frequency-scale, by curve C of Figure 2.For' purposes of explanation, it is assumed the side-band frequencygenerated by mixer IIA fallson point FAN of the righthand slope of thecurve when the operating frequency of oscillator` I IJ is at its desiredvalue. The side-band energy in its transmission through the gas cell ISAis subjectto;V attenuation and phase-shift predetermined by themolecular resonance curve C. Q'I'he gas cell I5A may, as more fullydescribed in copending application Serial No; 786,736, filed November18, 1947', comprise a resonant chamber or a section of waveguideAhavinggas-tight windows at opposite ends `thereof of mica or othersuitablematerial providing a gastight seal which is transparent vtomicrowave energy. The side-band energy as transmitted through gas cellI5A is impressed upon a demodulator I'IA, such as a crystal rectifier orequivalent, to produce a unidirectional voltage of magnitude dependentupon the amplitude of the transmitted side band. Assome of'the'gas lineshave an equivalent Q of about 70,000 the frequency-amplitudecharacteristic of the transmission through the gas is verysensitive tofrequency changes. As indicated in Figure 2', a minute change infrequency causes the amplitude abruptly to rise from FA to point FAL orabruptly to fall to point FAH.

Preferably, as indicated in Figure l, matching transformers IBA andl I6Bare interposed between the gas cells and their associated demodulatorsIIAand I'IB. I, n

In channel B," the output frequency of the microwave oscillator I0 isimpressed upon a mixer IIB upon which' is also'impressed the output of alow-frequency modulator IZB similar to oscillator I2A but whosefrequency Q 2x' l 5 h i is so chosen that one-of theside-bandfrequencies I wel-a w-a l i or 4 normally falls at point FBN on thelefthand slope of the gas-absorption line C, Figure 2. The two side-bandfrequencies generated by mixer IIB are propagated in both directionsfrom the mixer in channel B but attenuator IIIB prevents orminimizesrtheir interaction with the side bands generated. in channelA." As in channel A, an impedance-matching transformer I3B is preferablyinterposed between mixer IIB and the preceding portion of thetransmission line. The

output of mixer IIB is impressed upon the gas side band, for examplewhich falls uponthe slope of the absorption line C being subjected tolarge changes in attenuation and phase-shift as the microwave oscillatorfrequency cell I5B, the selected deviates from the desired. value.Specifically, in the case assumed, when the oscillator frequency risesbut to slight extent, the amplitude of the selected side-band energy inchannel B as transmitted by gas cell ISB rises abruptly from point FBN,Figure 2, to point Fan and abruptly falls to point'FBr. for slightdecrease in the frequency of oscillator I0. l

It is particularly to be noted that for an increase, for example, in thefrequency of oscillator II), the output of channel B abruptly rises topoint FBH whereas the output of channel A abruptly falls to point Fm. Onthe contrary, for slight decrease of oscillator III, the output ofchannel B abruptly falls toward point FBL and the output of. channelFAabruptly rises to point FAL. Thus a change in frequency of oscillatorI0 differentially affects the amplitudes of theoutputs of channels VAand B effectively doubling` the sensitivity already high because of theextreme steepness of the sides of the molecular resonance curve.

As indicated in Figure l, the demodulators IlA and I'IB are so poledthat their unidirectional currents are additive in comparator networkIBM which includes the resistors ISA-ISB connected in series with thedemodulatorsand a resistor 20 connected to the common terminal ofresistors ISA, I9B.

The resistances of resistors 4IIA--IEIB are preferably equal and verylarge compared to the resistance of resistor 20 to minimize reactionbetween the demodulators. ,'At the desired operating frequency ofoscillator I0, the outputs of the two channels are equal and'opposlteandr consequently the frequency-control voltage appearing betweenconductor 2| and ground, or other return-circuit, is zero. When thefrequency of oscillator In increases or decreases from the desiredvalue, the polarity and magnitude of the unidirectional potential'derived from the outputs Yof channels A,`B and impressed on conductor2| will depend upon the. sense and extent of the frequency-shift. "Theerror-control voltage so derived may be appliedinkn'own manner to themicrowave oscillator for stabilization of. its operating frequency: forexample, if the, oscillator I0 is a klystron, the controlvoltage may beapplied to vary the potential of the reflector electrode generally asshown in Figure 6 of aforesaid application Serial No. 35,185, jor if theoscillator I0 is replacedl by a single modulating oscillator, as inFigurev, by using a balanced magic T modulator similar to that shown inFigure 3 of my copending' application Serial No. 35,185. In such aIbalancedfI modulator. because of the iield'sym'- metry, there is nopropagation of side-band energy back into the microwave oscillatorchannel and consequently no need for any equivalent of attenuators MAand IAB.

Except in respects as specifically discussed, the system of Figure 3 issimilar to that of Figure 1: accordingly, corresponding elements havebeen identified by similar reference characteristics and the descriptionof Figure 3 principally directed to its differences.' As abovementioned, the side-A band energies respectively lying on opposite sidesof the sharp molecular resonance characteristic of the gas incellsIEA-IEB are transmitted in channels A and B with phase-angle shiftsdependent upon the frequency of oscillator I0, but as the frequenciesare different, these phase-shifts in the outputs of the gas cells ISA,I5B cannot directly be compared for control of the frequency ofoscillator I0. To provide for such comparison, there is included in thesystem of Figure 3 a beat-frequency oscillator 23 or equivalent forproducing oscillations whose frequency is equal tor the differencebetween the selected pair of side-band frequencies.

vIf the desired output frequency is lower than the center frequency ofthe selected gas reso` nance line, then for 3K@ side-band pairs are usedin channels A and B if the output frequency is within the interval ofthe gas line. If the outputfrequency is outside the interval of the gasline then only side-band pairs are used in the two channels.

Conversely, for aJ desired outputl frequency higher than the vselectedgas absorption line frequency, the pairs of side bands for channels Aand B are interval of the gas line. YThis beat-frequency and the lowerside-bandfrequency in channel B are whose phase differs therefrom-inaccordancel with the variations inf frequency 'of oscillator I0.`

The sign and magnitude ofthe phase angle between the outputsofthe 'twochannels may be measured by any suitablecomparatorV ISP, such as shown,for example, in my copending applications Serial Nos. 8,246 and 6,975 toproduce a frequency-control voltagetransmitted by conductor 2'I to theoscillator .IIJQQ Specifically, the output of one of thechanriels,"`fory example, channel A, may be converted to pairs of sharppulses applied to the rectifier bridge 24 comprising two pairs of.rectii'lers 25--25, 26,-26 poled as indicated sawtooth generatingcircuit, Inot shown, whose output is impressed upon the rectifier bridge24 and a network including coupling condenser 30, resistors 3|, 32 andbypass condenser 33. As more fully explained in my aforesaid copendingapplication, there is v'thus produced a unidirectional voltage varyingin sense and magnitude with the phaseang'le between the outputs of thetwo channels. lThis resulting unidirectional voltage may be applied byconductor 2| to vary the frequency of oscillator I0, generally as abovedescribed, in compensation for any deviations from its desired operatingfrequency.

The foregoing will be more fully understood from the followingmathematical discussion:

As already stated, either the side band m w-v 21r 21r l i produced inchannel A, is chosen to fall on one slope of the gas line; assuming theformer, the eld propagated through the gas may be expressed as:

where a is an arbitrary phase angle.

The propagation at the other side-band frequency, specifically will beat constant amplitude since it is not withinthe frequency interval'ofthe resonance curve. Moreover if there is some transmission of thisfrequency 4through the gas, there is no'adverse effect because thephase-frequency characteristic atfrequencies substantially higher andlower than'the peak is essentially flat for small frequency increments.lIn other words, the non-selected side band will be propagated withpractically constant phase for normally encountered frequency variationsof the stabilized oscillator Ill. The selected side-band propagation iscombined with propagation at vthe carrier frequency which may beexpressed as I v e2=E2 sin wt (2) If the mixer I5A or I5B has asquare-law characteristic, its output may therefore be expressed asimpressed uponl a mixer 2 2 whose output therefor `includes a frequencywhich is equal to the higher side-band frequency in channel A but wherevA i y m=inodulation factor =phaseshiftdue togas resonance.Y

acoger? If the demodulatorjllA is notsquare-law. additional higherorder'terms will appear. I-Iowever, they are effectively removed byfilter 35A which passes only the frequency In similar manner, v'theoscillator 12B is closely regulated to producemodulating frequency whichis impressed upon the mixer Hl?.v to produce side bands 1 croi." .c-d

. ...2111. and' .2r

one of which fallso'rrv the other Vslope of the resonance curve of thegas;it-is assumed for simplicity of explanation that the former falls onthe lefthandr slope'of the gas line, Figure 2. vThe mixer output maytherefore be expressed as where .ai is an arbitrary phase angle.

This side-band propagation is combined with the carrier propagation andthe resultant fields are applied to the 'demodulator' ITB whose outputmay therefore, assuming a square-law response, be expressed as modulatorI'Vinte'rpsedfbetween the gas cell l5 or equivalent'ad the microwaveoscillator l0. The output offthe' balanced modulator Il thereforecomprises twoside-band frequencies only, theV vcarrier frequency beingsuppressed. The frequency are subjected to attenuations and phase-shiftsdifferentially in accordance with deviations of the frequency vofoscillator l0 from the desired operating frequency,

It is convenientl to connect the same beat oscillator 2 3 to the mixerto beat the two side bands to lower frequencies where phase or amplitudecomparison is easier. These two side bands so where m is the modulationAfactor is the phase-shift due to the gas.

The outputs of the twoY channels may therefore be expressed as:

(Channel Ai' @Frs cos @www (Channel B) ec=jEs ZOS `(\,bl'-a1ii) Thephase angles a and a1 are constant, whereas the phase angles and phwhich are opposite in algebraic sign since they are derived fromfrequency changes on opposite slopes of the same resonancecharacteristic, depend upon the frequency-phase characteristics of thegas and are subject to large variation for small deviations in frequencyof oscillator I0.

The frequency-stabilizing system shown in Figure 5 is similar to that ofFigure 3'except that the beat-frequency applied to mixer. 22'is notproduced by an additional oscillator 23. Instead, the outputs of themodulating oscillators IZA, 12B are impressed upon a mixer 34 of anysuitable type so to produce a beat-frequency equal to the differencebetween-the two chosen side-band'frequencies. The beat-frequency appliedto mixer. 22 of Figure 5 is therefore always equalto the difference inside-band frequencies notwithstandingy any instability of either or bothof the modulating oscillators I2A, IZB'. In this respect the systemofFigure is tobe preferred, as it is not necessary so rigidly to controlthe frequency of the modulating oscillators.

Further explanation of the system of Figure 5 appears unnecessary,v asthe corresponding 'elements of both systems are identified by similarreference characters and the previous general description of Figure 3directly applies to Figure 5.

In the system shown in Figure 6, which is somewhat simpler than thatofpreceding modifications, the output or a single low-frequencymodulating oscillator l2 is impressed upon a balanced transmitted areselected by lters 35A, 35B and either the difference in their amplitudesor the difference in their phase-angle may 4be measured by comparator I8and used to regulate' the frequency of oscillator l0 as above describedin connection with Figure 1 for amplitude comparison or Figures Brand 5for phase comparison.

When phasev comparison. control is employed, there is additionallyrequired, as in Figure 3, a mixer 24 and a source of voltage at twicethe frequency of oscillator I2; The double frequency voltage may beobtained from a frequency multiplier 25 as shown or from a separateoscillator. In either case the Afrequency of the potential suppliedtomixer 24is equal to the difference between the side-band frequenciesso that the outputs of lters 35A and 35B' are two equal frequencies witha phase-difference differentially varying with deviation from normal ofthe frequency of microwave oscillator l0, generally as discussed inconnection with Figures 3 and 5. It shall be understood the invention isnot limited to the exemplary systems specically described and thatchanges and modifications may be made within the scope of the appendedclaims.

What is claimed isi l. The method of stabilizing the operating frequencyof a microwave generator yby utilization of a sharp molecular resonancecharacteristic of a gas which comprises modulating the oscillatlonsproduced by said generator to produce sideband frequencieson oppositeslopes of said resonance characteristic, transmitting said side bandsthroughsaid gas, demodulating the transmitted yside bands, and derivingfrom the demodulated side bands a frequency-control signal applied tosaid oscillator in compensation for deviations from its desiredoperating frequency.

2. The method of stabilizing the frequency o1' a microwave' generator byutilization4 of a sharp molecular resonance characteristic of a gaswhich comprises modulating the oscillations produced by said generatorto produce side-band frequen- 9 Y cies on opposite slopes of saidresonance characteristic, transmitting saidside bands through said gasdifferentially to'vary'their amplitudes upon deviation from the desiredfrequency` of theL generated oscillations, demodulating said sidebandsas transmitted by said gas, deriving from the demodulated side .bands aunidirectional voltage whose amplitude varies in sense and to extentdependent upon. the sense and extent ofy L deviations ofthe generatorlfrequency, and applying said unidirectional voltage to said generatorfto correct for said deviations of its frequency.

3. The method of stabilizing the frequency of a microwave generator byutilization of asharp molecular resonance characteristic of a gas whichcomprises modulating the oscillations produced by said generator toproduce side-band frequencies on opposite slopes of said resonancecharacteristic, transmitting said side-band frequencies through said gasto shift. their phases in accordance with deviations from the desiredoperating frequency of said generator, mixing kone of said side-bandfrequencies with oscillations of frequency equal to the differencebetween said side-band frequencies to produce a frequency equal to theunmixed side-band lfrequency and with a phase-difference correspondingwith the sense of deviation of the generator frequency, and varying afrequency-control voltage of said generator in accordance with saidphase difference.

4. The method of stabilizing the operating frequency of a microwaveoscillator by utilization of the sharp molecular resonancecharacteristic of a. gas absorption line, which comprises impressingoutput energy from the oscillator upon transmission channels eachincluding gas exhibiting molecular resonance at the desired operatingfrequency, mixing the oscillator energy in each channel with the outputof a stable low-frequency oscillator to produce side-band frequencies onopposite slopes of said gas resonance characteristic, demodulating theside bands as transmitted .by said gas, and deriving from thedemodulated side bands a unidirectional frequency-control signal appliedto said oscillator. Y

5. The method of stabilizing the operating frequency of a microwaveoscillator by utilization of the sharp molecular resonancecharacteristic of a gas which comprises impressing output energy of theoscillator upon transmission channels each including gas exhibitingmolecular resonance, mixing the oscillator energy in said channels inadvance of the gas with low-frequency oscillations to produce side-bandfrequencies respectively on opposite sides of said resonancecharacteristic, demodulating the side bands as transmitted by the gas toproduce unidirectional voltages varying differentially With deviation ofthe operating frequency, and'applying the resultant of saidunidirectional voltages to said oscillator for controlof its operatingfrequency.

6. The method of stabilizing the operating frequency of a microwaveoscillator by utilization of a sharp molecular resonance characteristicof a gas which comprises impressing output energy of the oscillatorenergyupon transmission channels each including gas exhibiting molecularresonance, in each channel and in advance ofy said gas modulating theoscillator energy to produce side-band frequencies on opposite slopes ofsaid resonance characteristic, mixing one of said sideband frequenciesas transmitted by the gas with oscillations of frequency .equal to thedifference between said side-band frequencies to produce a I0 frequencyequal totheunmixed sideband fre-1 quency and with `a phase-differencedependent upon deviation fromf the desired cperatingfre'- quency, and j/arying a frequency-control' voltage of said oscillator in accordancewith said phasedifference. l

7. The method of stabilizingthe operating frequency of a microwavegenerator by utilization of a sharp molecular-resonancecharacteristic'of a gas which comprises impressing 4output'energy of theoscillator upon transmission channels 4including gas' exhibitingymolecular resonance,k in each channelfin advance of :the gas mixing Ytheoutputs of said generator and a lowfrequency oscillator to produceside-band frequenciesin different channels which are respectivelyon op-.posite slopes of said resonance"characteristic,

mixing the outputs of said low-frequency oscillators to produce abeat-frequency, mixing said beat-frequency and one of2 said side-band frequencies as transmitted by the vgas, to produce. a frequency equal tothe' unmixed side-band frequency and with a phase-differencecorresponding with deviation fromfthe desired operating frequency, andVarying a frequency-control voltage of said oscillatorin accordancewitht said phase-difference.v f. 'I

8. The method of stabilizing lthe operatingffrequency of a microwavelgenerator by utilizationof the sharpmolecular.resonance.'characteristic of a gas which, comprisesimpressingtheoutput signals of ysaid generatorfandallow-frequencyoscillator uponv a balanced modulatorto produce side-bands on opposite.slopes of 7said resonance characteristic, impressingsaid side-bands uponsaid gas, mixing the side bands` as transmitted by said gas withoscillations of frequency equal to the difference between them to.produce outputs of similar frequency but differing in phase andamplitude in accordancewith deviation Vfrom the desired operating.Ifrequency, and varyingfa frequency-control voltage of `said generator inaccordance Ywith the difference between said outputs.l ,v

9. The method of stabilizing the' operating frequency of an oscillation`generatorby utilization of the sharp. resonance characteristicof a high-Q circuit element whichcomprises modulating the generated oscillationsto produce side-bandire,- quencieson opposite slopes of said resonancecharacteristic, impressing; the side-bands upon said circuit element fortransmission thereof with attenuations and phase-shifts differentially,varying in accordance 'with 'deviation from the desired operatingfrequency .o f said generator, demodulating the transmitted sidewbands,l j-and varying the frequency of saidl generator in ,accordancewith demodulationvcomponents` of the side bands in-compensation'- for;deviations from I 10. YA system for stabilizing theioperating frequencyofA a microwave generator byutilizatio'n of a sharp molecular resonancecharacteristicof a gas `which comprises means-for'modulating theoscillations'produced by said generator'to produce quency of a microwavegenerator comprising two 1l trensmssionehannels each-including a CellC011- taining gas haring a sharp molecular resonance characteristic anda mixer between said cell and said generator, low frequency oscillatorsrespectively connected `to said mixers to produce sideband frequenciesrespectively on opposite slopes of said resonance characteristic andrespectively transmitted by v-said cells with attenuationsdifferentially dependent upon the sense of deviation from the desiredoperating frequency, means including demodulators in said channelsbeyond the respective gas cells for producing channel outputs -whoseamplitudes differentially vary withdeviation of theoperating frequency,and control means for Varying the frequency of said generator inaccordance with the sense and magnitude of the difference in amplitudesof said channel outputs.

12. A system for stabilizing the operating frequency of amicrowavegenerator comprising .two transmission channels each includinga cell containing gas having a sharp molecular resonance characteristicand a mixer between said cell and said generator, low-frequencyoscillators respectively connected to said mixers to produce sidebandfrequencies respectively on opposite slopes of said resonancecharacteristic and respectively transmitted by said cells withphaseeshifts differentially dependent upon the sense of deviation fromthe desired yoperating frequency, means including demodulators in saidchannels beyond the respective gas cells for producing channel outputswhose phase-difference corresponds with deviations from `the desiredoperating frequency, and control means for correcting the frequency cfsaid generator in accordance with variations of Said phasedference y,1.3. A system rfor ,stabilizing the operating frequency of a 1microwavegenerator comprising two transmission channels each including a cellcontaining gas having a sharp molecular resonance characteristic and amixer between said cell and said generaton'loW-frequency oscillatorsrespectively connected to said `mixers to produce side-band frequenciesrespectively on opposite slopes of said resonance characteristic andrespectively transmitted with phase shifts differentially dependent uponthe sense of deviation from the desired operating frequency. means forproducing oscillations whose frequency equals the difference betweensaid side-band frequencies, mixing means upon which the lower of saidside bands and said oscillations are impressed to produce intheqcorresponding channel an output frequency equal to the outputfrequency of the other channel, and control means for regulating theoperating frequency of said generator in accordance with thedifferencein phase of the outputs of saidchannels.

14. A system for stabilizing the operating frequency of a microwavegenerator comprising two transmission channels each including a cellcontaining gas having a sharp molecular resonance characteristic and amixer between said cell and said generator, low-frequency oscillatorsrespectively connected to said mixers to produce sideband frequenciesrespectively on opposite slopes of said resonance characteristic andrespectively transmitted with phase-shifts differentially dependent uponthe sense of deviation from the desired operating frequency, a beatoscillator whose frequency equals the difference between said sidebandfrequencies, mixing means upon which the lower of said side-bandfrequenciesv and the beatfrequency are impressed to produce in thecorresponding channel .Ian output frequency equal to the outputfrequency of theother channel, and control means lfor regulating theoperating frequencyl of said microwave generator inaccordance with thevdifference in phase of the outputs of said channels.

15. A system for stabilizing the operating frequency of a microwavegenerator comprising two transmission channels each including a cellcontaining gas'having a sharp molecular resonance characteristic andafrnixer between said cell and said generator, low frequency oscillatorsrespectively connected to said mixers .to produce sideband frequenciesrespectively on opposite slopes of said resonance characteristic andrespectively transmitted with phase-.shifts differentially dependentupon the sense of deviation from the dcsired operating frequency, mixingmeansA connected to said low-frequency voscillators to produce abeat-frequency equal to the difference between said side-bandfrequencies, mixing means upon which the beat-frequency andthe lower ofsaid side-band frequencies are impressedv to produce in thecorresponding channel an output frequency equal to the output frequencyof the other channel, and controll means for regulating the operatingfrequency of said microwave generator in accordance with the differencein phase of the outputs of said channels.l

16. A system forstabilizing the operating fre-- quency of a microwavegenerator comprising a transmission -channel including a cellcontaining. gashaving 'a sharp molecular resonance characteristic, abalanced modulator included in said channel betweensaid gas cell andsaid microwave generator, a low-frequency oscillator connected to saidmodulator to Yproduce side-band frequencies respectively'on Oppositeslopes of said reso-` nance characteristic larid transmitted by said gaswith attenuations and phase-shifts differentially varying with deviation'from the desired operating frequency of said microwave generator, amixer upon which the transmitted side-bands are impressed, a'beat-frequency oscillator connected to said mixer to produce outputcomponents of said channel which 4are of the same frequency but differin phase and amplitude, and control means for regulating the operatingfrequency of said microwave generator in accordance withdifferencesbetween said output componente.` Y

1'7. A system for stabilizing the operating frequency of a microwavegenerator comprising a transmission channel including a cell containinggas having a sharp molecular resonance characteristic, a balancedmodulator included in said channel between said gas cell and saidmicrowave generator, a low-frequencyv oscillator connected to saidmodulator to produce side-band frequencies respectively on oppositeslopes of said resonance characteristic and transmitted by said gas withattenuations and phase-shifts` differentially varying with deviationfrom the dcsired operating frequency of said microwave generator, amixer upon which the transmitted sidebands are impressed, abeat-frequency oscillator connected to said mixer to produce outputcornponents of said channel which are of the same frequency but differsubstantially only in phase, and control means for regulating theoperating frequency of said microwave generator in accordance withdifferences between said output components.

18. A system for'stabilizing the operating frcquency of a microwavegenerator comprising a transmission channel including a cell containinggas having a sharp molecular resonance characteristic, a balancedmodulator included in said channel between said gas cell and saidmicrowave generator, a low-frequency oscillator connected to saidmodulator to produce side-band frequencies respectively on oppositeslopes of said resonance characteristic and transmitted 'by said gaswith attenuations and phase-shifts diierentially varying with deviationfrom the desired operating frequency of said microwave generator, amixer upon which the transmitted side-bands are impressed, means forproviding control signals to provide output signal components whichdiffer substantially only in amplitude, and control means for regulatingthe operating frequency of said microwave generator in accordance withdifferences between said output components.

19. A system for stabilizing the operating frequency of a microwavegenerator comprising two transmission Ichannels at least one channelincluding a cell containing gas having a sharp molecular resonancecharacteristic and a mixer between said cell and said generator, a lowfrequency oscillator connected to said mixer to prowith the differencein phase of the outputs of said channels.

LOWELL E. NORTON.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS yNumber Name Date 1,889,083 Wintringham Nov. 29,1932 2,425,981 Bard et al Aug. 19, 1947

